Conventionally, as a technology for producing a fine wiring pattern, a method for forming a wiring pattern by combining a copper foil and photoresist and using lithography, has been generally used. However, this method has a large number of steps, and puts a large burden on drainage and liquid waste disposal. Thus, improvement in terms of environment has been desired. In addition, a method for patterning a metal thin film made by heating deposition or sputtering, using a photolithography method, is also known. However, a vacuum environment is necessary for the heating deposition and sputtering, and the cost therefor is very expensive. Thus, when this method is applied to form a wiring pattern, reduction of the production cost is quite difficult.
Thus, a technology for producing a wiring pattern by printing, using metal ink (including ink in which an oxide is metallized by reducing with a reducing agent), has been proposed. The wiring technology by printing enables the production of a large amount of products at a low cost and at high speed, and thus, the technology has already been examined for producing a practical electronic device.
However, the method for heating and sintering metal ink using a heating furnace has drawbacks that a heating process takes time, and that when a plastic substrate is not resistant to the heating temperature necessary for sintering the metal ink, sintering has to be performed at a temperature the plastic substrate is resistant, and thus, the conductivity cannot reach a satisfactory level.
As described in Patent Documents 1 to 3, using a composition (ink) including nano-particles, and transforming the ink to metal wiring by photo irradiation have been tried.
The method using optical energy or microwave for heating is a very good method because only ink portions can be heated. However, the method has drawbacks that when metal particles themselves are used, the conductivity of the obtained conductive pattern cannot be increased to a satisfactory level, and when a copper oxide is used, the obtained conductive pattern has a large void ratio and a part of the copper oxide particles remain unreduced.
Further, there are further drawbacks that metal or metal oxide particles having a diameter of 1 μm or less should be used for sintering, and preparing such nano-particles requires a very high cost.